Etching method

ABSTRACT

There is provided a method for selectively etching a first region of silicon oxide with respect to a second region of silicon nitride by performing plasma processing on a target object including the second region formed to have a recess, the first region provided to fill the recess and to cover the second region, and a mask provided on the first region. The method includes: (a) generating a plasma of a processing gas containing a fluorocarbon gas in a processing chamber where the target object is accommodated and forming a deposit containing fluorocarbon on the target object; (b) generating a plasma of a processing gas containing an oxygen-containing gas and an inert gas in the processing chamber; and (c) etching the first region by radicals of fluorocarbon contained in the deposit. A sequence including the step (a), the step (b) and the step (c) is repeatedly performed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2015-006775 filed on Jan. 16, 2015 and Japanese Patent Application No. 2015-034212 filed on Feb. 24, 2015, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The disclosure relates to an etching method, and more particularly, to a method for selectively etching a first region of silicon oxide with respect to a second region of silicon nitride by performing plasma processing on a target object.

BACKGROUND OF THE INVENTION

In manufacturing electronic devices, there is performed a process of forming an opening such as a hole, a trench at a region of silicon oxide (SiO₂). In such a process, a target object is exposed to a plasma of a fluorocarbon gas and, thus, the region of silicon oxide is etched, as disclosed in U.S. Pat. No. 7,708,859.

There is known a technique for selectively etching a first region of silicon oxide with respect to a second region of silicon nitride. A SAC (Self-Aligned Contact) technique is known as an example of such a technique. The SAC technique is disclosed in Japanese Patent Application Publication No. 2000-307001.

The target object as an object to be processed by the SAC technique includes a first region of silicon oxide, a second region of silicon nitride, and a mask. The second region is formed to have a recess. The first region is formed to fill the recess and cover the second region. The mask is provided on the first region and has an opening provided over the recess. As disclosed in Japanese Patent Application Publication No. 2000-307001, in the conventional SAC technique, a plasma of a processing gas containing a fluorocarbon gas, an oxygen gas and an inert gas is used to etch the first region. By exposing the target object to the plasma of the processing gas, a portion of the first region which is exposed through the opening of the mask is etched. As a consequence, an upper opening is formed. Further, by exposing the target object to the plasma of the processing gas, a portion surrounded by the second region, i.e., the first region in the recess, is etched in a self-aligned manner. Accordingly, a lower opening continuous to the upper opening is formed in a self-aligned manner.

In the above conventional technique, as the etching of the first region proceeds, an opening of the mask and/or an opening formed by etching the first region are narrowed by the deposit originated from fluorocarbon and this may result in blockage of the openings. As a result, the etching rate of the first region is decreased and the etching of the first region may be stopped in some cases.

Therefore, it is required to etch the first region of silicon oxide with respect to the second region of silicon nitride while preventing blockage of the openings.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, there is provided a method for selectively etching a first region of silicon oxide with respect to a second region of silicon nitride by performing plasma processing on a target object including the second region formed to have a recess, the first region provided to fill the recess and to cover the second region, and a mask provided on the first region, the method including the steps of: (a) generating a plasma of a processing gas containing a fluorocarbon gas in a processing chamber where the target object is accommodated and forming a deposit containing fluorocarbon on the target object; (b) generating a plasma of a processing gas containing an oxygen-containing gas and an inert gas in the processing chamber; and (c) etching the first region by radicals of fluorocarbon contained in the deposit, wherein a sequence including the step (a), the step (b) and the step (c) is repeatedly performed.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the disclosure will become apparent from the following description of embodiments, given in conjunction with the accompanying drawings, in which:

FIG. 1 is a flowchart of an etching method according to an embodiment;

FIG. 2 is a cross sectional view showing an example of a target object to which the etching method according to the embodiment is applied;

FIG. 3 schematically shows an example of a plasma processing apparatus capable of performing the method shown in FIG. 1; and

FIGS. 4 to 16 are cross sectional views showing the target object in respective steps of the etching method shown in FIG. 1.

FIG. 17 is a flowchart of an etching method according to another embodiment; and

FIGS. 18 and 19 are cross sectional views showing the state of the target object after execution of a step ST14 of the method shown in FIG. 17.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments will be described with reference to the accompanying drawings which form a part hereof. Like reference numerals will be used for like parts throughout the drawings.

FIG. 1 is a flowchart of an etching method according to an embodiment. A method MT shown in FIG. 1 is an etching method for selectively etching a first region of silicon oxide with respect to a second region of silicon nitride by performing plasma processing on a target object.

FIG. 2 is a cross sectional view showing an example of a target object to which the etching method according to the embodiment is applied. As shown in FIG. 2, the target object, i.e., a wafer W, includes a substrate SB, a first region R1, a second region R2, and an organic film OL that will becomes a mask later. For example, the wafer W is obtained during the manufacturing process of a fin-type field effect transistor and further includes a protruding region RA, a silicon-containing anti-reflection film AL, and a resist mask RM.

The protruding region RA protrudes from the substrate SB. The protruding region RA may serve as, e.g., a gate region. The second region R2 is made of silicon nitride (Si₃N₄) and formed on surfaces of the protruding region RA and the substrate SB. As shown in FIG. 2, the second region R2 extends to form a recess. For example, the recess has a depth of about 150 nm and a width of about 20 nm.

The first region R1 is made of silicon oxide (SiO₂) and formed on the second region R2. Specifically, the first region R1 fills the recess formed by the second region R2 and covers the second region R2.

The organic film OL is formed on the first region R1. The organic film OL is made of an organic material, e.g., amorphous carbon. The anti-reflection film AL is formed on the organic film OL. The resist mask RM is formed on the anti-reflection film AL. The resist mask RM provides an opening having a width greater than that of the recess formed by the second region R2. The opening of the resist mask RM has a width of, e.g., about 60 nm. A pattern of the resist mask RM is formed by a photolithography technique.

In the method MT, the target object such as the wafer W shown in FIG. 2 is processed in a plasma processing apparatus. FIG. 3 schematically shows an example of a plasma processing apparatus capable of performing the method shown in FIG. 1. A plasma processing apparatus 10 shown in FIG. 3 is configured as a capacitively coupled plasma etching apparatus and includes a substantially cylindrical processing chamber 12. An inner wall surface of the processing chamber 12 is made of, e.g., anodized aluminum. The processing chamber 12 is frame-ground.

A substantially cylindrical supporting part 14 is formed at a bottom portion of the processing chamber 12. The supporting part 14 is made of, e.g., an insulating material. In the processing chamber 12, the supporting part 14 extends vertically from the bottom portion of the processing chamber 12. Further, in the processing chamber 12, a mounting table PD is provided. The mounting table PD is supported by the supporting part 14.

The wafer W is held on a top surface of the mounting table PD. The mounting table PD includes a lower electrode LE and an electrostatic chuck ESC. The lower electrode LE has a first plate 18 a and a second plate 18 b. The first plate 18 a and the second plate 18 b are made of a metal, e.g., aluminum, and have a substantially disc shape. The second plate 18 b is provided on the first plate 18 a and electrically connected to the first plate 18 a.

The electrostatic chuck ESC is provided on the second plate 18 b. The electrostatic chuck ESC has a structure in which an electrode made of a conductive film is interposed between two insulating layers or two insulating sheets. A DC power supply 22 is electrically connected to the electrode of the electrostatic chuck ESC through a switch 23. The wafer W is attracted on the electrostatic chuck ESC by an electrostatic force such as a Coulomb force or the like generated by a DC voltage applied from the DC power supply 22. Thus, the electrostatic chuck ESC is capable of holding the wafer W thereon.

A focus ring FR is provided on a peripheral portion of the second plate 18 b to surround the edge of the wafer W and the electrostatic chuck ESC. The focus ring FR is provided to improve the in-plane uniformity of the etching. The focus ring FR is made of a material that is appropriately selected depending on a material of an etching target film. For example, the focus ring FR may be made of quartz.

A coolant path 24 is provided in the second plate 18 b. The coolant path 24 constitutes a temperature control mechanism. A coolant is supplied to the coolant path 24 from an external chiller unit through a line 26 a. The coolant flowing in the coolant path 24 returns to the chiller unit through a line 26 b. The coolant circulates between the coolant path 24 and the chiller. A temperature of the wafer W held on the electrostatic chuck ESC is controlled by controlling a temperature of the coolant.

The plasma processing apparatus 10 further includes a gas supply line 28. A heat transfer gas supply unit (not shown) supplies a heat transfer gas, e.g., He gas to a gap between a top surface of the electrostatic chuck ESC and a backside of the wafer W through the gas supply line 28.

The plasma processing apparatus 10 further includes an upper electrode 30. The upper electrode 30 is provided above the mounting table PD to face the mounting table PD. The upper electrode 30 and the lower electrode LE are approximately parallel to each other. Between the upper electrode 30 and the lower electrode LE, a processing space S where plasma processing is performed in the wafer W is defined.

The upper electrode 30 is held at an upper portion of the processing chamber 12 through an insulating shield member 32. In the present embodiment, a vertical distance from a top surface of the mounting table PD, i.e., a wafer mounting surface, to the upper electrode 30 may be variable. The upper electrode 30 may include an electrode plate 34 and an electrode plate holder 36. The electrode plate 34 is in contact with the space S and has a plurality of gas injection holes 34 a. In the present embodiment, the electrode plate 34 is made of silicon.

The electrode plate holder 36 detachably holds the electrode plate 34 and is made of a conductive material, e.g., aluminum. The electrode plate holder 36 may have a water cooling structure. A gas diffusion space 36 a is provided in the electrode plate holder 36. A plurality of gas holes 36 b communicating with the gas injection holes 34 a extends downward from the gas diffusion space 36 a. Further, the electrode plate holder 36 includes a gas inlet port 36 c for guiding a processing gas into the gas diffusion space 36 a. A gas supply line 38 is connected to the gas inlet port 36 c.

The gas supply line 38 is connected to a gas source group 40 through a valve group 42 and a flow rate controller group 44. The gas source group 40 includes a plurality of gas sources. In this example, the gas source group 40 includes one or more fluorocarbon gas sources, a rare gas source, a nitrogen gas (N₂ gas) source, a hydrogen gas (H₂ gas) source, and an oxygen-containing gas source. One or more fluorocarbon gas sources may include a C₄F₈ gas source, a CF₄ gas source, and a C₄F₆ gas source. The rare gas source may be a source of any rare gas such as He gas, Ne gas, Ar gas, Kr gas, Xe gas or the like. In this example, the rare gas source may be an Ar gas source. In this example, the oxygen-containing gas source may be an oxygen gas (O₂ gas) source. The oxygen-containing gas may also be any gas containing oxygen, e.g., a carbon oxide gas such as CO gas or CO₂ gas.

The valve group 42 includes a plurality of valves. The flow rate controller group 44 includes a plurality of flow rate controllers such as a mass flow rate controller and the like. The gas sources of the gas source group 40 are connected to the gas supply line 38 through corresponding valves of the valve group 42 and corresponding flow rate controllers of the flow rate controller group 44, respectively.

In the plasma processing apparatus 10, a deposition shield 46 is detachably provided along the inner wall of the processing chamber 12. The deposition shield 46 is also provided at an outer periphery of the supporting part 14. The deposition shield 46 prevents an etching by-product (deposit) from being adhered to the processing chamber 12. The deposition shield 46 may be made of aluminum coated with ceramic such as Y₂O₃ or the like.

A gas exhaust plate 48 is provided at a lower portion in the processing chamber 12 and between the supporting part 14 and the sidewall of the processing chamber 12. The gas exhaust plate 48 may be formed by coating aluminum with ceramic, e.g., Y₂O₃ or the like. In the processing chamber 12, a gas exhaust port 12 e is provided below the gas exhaust plate 48. A gas exhaust unit 50 is connected to the gas exhaust port 12 e through a gas exhaust line 52. The gas exhaust unit 50 has a vacuum pump such as a turbo molecular pump or the like, so that a pressure in the space in the processing chamber 12 can be decreased to a predetermined vacuum level. A loading/unloading port 12 g for the wafer W is provided at the sidewall of the processing chamber 12. The loading/unloading port 12 g can be opened and closed by a gate valve 54.

The plasma processing apparatus 10 may further include a first high frequency power supply 62 and a second high frequency power supply 64. The first high frequency power supply 62 generates a high frequency power for plasma generation which has a frequency in a range from 27 MHz to 100 MHz. The first high frequency power supply 62 is connected to the upper electrode 30 through a matching unit 66. The matching unit 66 is a circuit for matching an output impedance of the first high frequency power supply 62 with an input impedance of the load side (the upper electrode 30 side). The first high frequency power supply 62 may be connected to the lower electrode LE through the matching unit 66.

The second high frequency power supply 64 generates a high frequency bias power for ion attraction which has a frequency in a range from 400 kHz to 13.56 MHz. The second high frequency power supply 64 is connected to the lower electrode LE through a matching unit 68. The matching unit is a circuit for matching an output impedance of the second high frequency power supply 64 with an input impedance of the load side (the lower electrode LE side).

The plasma processing apparatus 10 further includes a power supply 70. The power supply 70 is connected to the upper electrode 30. The power supply 70 applies to the upper electrode 30 a voltage for attracting positive ions in the processing space S to the electrode plate 34. In this example, the power supply 70 is a DC power supply for generating a negative DC voltage. In another example, the power supply 70 may be an AC power supply for generating an AC voltage having a relatively low frequency. The voltage applied from the power supply 70 to the upper electrode 30 may be lower than or equal to −150 V. In other words, the voltage applied from the power supply 70 to the upper electrode 30 may be a negative voltage having an absolute value of 150 or above. When such a voltage is applied from the power supply 70 to the upper electrode 30, the positive ions in the processing space S collide with the electrode plate 34. Accordingly, secondary electrons and/or silicon are emitted from the electrode plate 34. The emitted silicon is combined with active species of fluorine in the processing space S, so that the amount of active species of fluorine is decreased.

In the present embodiment, the plasma processing apparatus 10 may further include a control unit Cnt. The control unit Cnt is a computer including a processor, a storage unit, an input device, a display device and the like. The control unit Cnt controls the respective components of the plasma processing apparatus 10. The control unit Cnt can allow an operator to input commands to manage the plasma processing apparatus 10 by using the input device and display the operation state of the plasma processing apparatus 10 on the display device. The storage unit of the control unit Cnt stores therein a control program for controlling various processes performed in the plasma processing apparatus 10, and a program, i.e., a processing recipe, for performing processes of the respective components of the plasma processing apparatus 10 based on the processing conditions.

Herein after, referring back to FIG. 1, the method MT1 will be described in detail. FIGS. 2 and 4 to 16 will be appropriately referred to in the following description. FIGS. 4 to 16 are cross sectional views showing the target object in respective steps of the method MT. In the following description, there will be described an example in which the wafer W shown in FIG. 2 is processed by the plasma processing apparatus 10 shown in FIG. 3 according to the method MT.

In the method MT, first, the wafer W shown in FIG. 2 is loaded into the plasma processing apparatus 10 and then mounted and held on the mounting table PD.

Next, a step ST1 of the method MT is performed. In the step ST1, the anti-reflection film AL is etched. To do so, a processing gas is supplied into the processing chamber 12 from a gas source selected among the gas sources of the gas source group 40. The processing gas used in the step ST1 contains a fluorocarbon gas. The fluorocarbon gas may contain, e.g., at least either one of C₄F₈ gas and CF₄ gas. The processing gas may further contain a rare gas, e.g., Ar gas. In the step ST1, a pressure in the processing chamber 12 is set to a predetermined level by the operation of the gas exhaust unit 50. In the step ST1, the high frequency power from the first high frequency power supply 62 is supplied to the upper electrode 30 and the high frequency bias power from the second high frequency power supply 64 is supplied to the lower electrode LE.

The step ST1 is executed under the following condition.

-   -   Pressure in processing chamber: 10 mTorr (1.33 Pa) to 50 mTorr         (6.65 Pa)     -   Processing gas         -   C₄F₈ gas: 10 sccm to 30 sccm         -   CF₄ gas: 150 sccm to 300 sccm         -   Ar gas: 200 sccm to 500 sccm

High frequency power for plasma generation: 300 W to 1000 W

High frequency bias power: 200 W to 500 W

In the step ST1, the plasma of the processing gas is generated and a portion of the anti-reflection film AL which is exposed through the opening of the resist mask RM is etched by active species of fluorocarbon. As a result, the portion of the anti-reflection film AL which is exposed through the opening of the resist mask RM is removed as can be seen from FIG. 4. In other words, a pattern of the resist mask RM is transferred onto the anti-reflection film AL and the anti-reflection film AL is formed to have a pattern providing an opening. In the step ST1, the operation of each component of the plasma processing apparatus 10 can be controlled by the control unit Cnt.

Subsequently, in a step ST2, the organic film OL is etched. To do so, a processing gas is supplied into the processing chamber 12 from a gas source selected among the gas sources of the gas source group 40. The processing gas used in the step ST2 may contain hydrogen gas and nitrogen gas. The processing gas used in the step ST2 may contain another gas, e.g., oxygen gas, as long as it can etch the organic film. In the step ST2, a pressure in the processing chamber 12 is set to a predetermined level by the operation of the gas exhaust unit 50. In the step ST2, the high frequency power from the first high frequency power supply is supplied to the upper electrode 30 and the high frequency bias power from the second high frequency power supply 64 is supplied to the lower electrode LE.

The step ST2 is executed under the following condition.

-   -   Pressure in processing chamber: 50 mTorr (6.65 Pa) to 200 mTorr         (26.6 Pa)     -   Processing gas         -   N₂ gas: 200 sccm to 400 sccm         -   H₂ gas: 200 sccm to 400 sccm     -   High frequency power for plasma generation: 500 W to 2000 W     -   High frequency bias power: 200 W to 500 W

In the step ST2, the plasma of the processing gas is generated and a portion of the organic film OL which is exposed through the opening of the anti-reflection film AL is etched. The resist mask RM is also etched. As a result, the resist mask RM is removed and the portion of the organic film OL which is exposed through the opening of the anti-reflection film AL is removed, as can be seen from FIG. 5. In other words, the pattern of the anti-reflection film AL is transferred onto the organic film OL and the organic film OL is formed to have a pattern providing an opening MO, thereby serving as a mask MK. In the step ST2, the operation of each component of the plasma processing apparatus 10 can be controlled by the control unit Cnt.

In the present embodiment, a step ST3 is executed after the step ST2. In the step ST3, the first region is etched. The etching of the first region R1 is stopped immediately before the second region R2 is exposed. In other words, the first region R1 is etched until the first region R1 remains a little on the second region R2. To do so, in the step ST3, a processing gas is supplied into the processing chamber 12 from a gas source selected among the gas sources of the gas source group 40. The processing gas used in the step ST3 contains a fluorocarbon gas. Moreover, the processing gas may further contain a rare gas, e.g., Ar gas. Further, the processing gas may contain oxygen gas. In the step ST3, a pressure in the processing chamber 12 is set to a predetermined level by the operation of the gas exhaust unit 50. In the step ST3, the high frequency power supply from the first high frequency power supply 62 is supplied to the upper electrode 30 and the high frequency bias power from the second high frequency power supply 64 is supplied to the lower electrode LE.

In the step ST3, the plasma of the processing gas is generated and a portion of the first region R1 which is exposed through the opening of the mask MK is etched by active species of fluorocarbon. The processing time of the step ST3 is set such that the etched first region R1 having a predetermined film thickness remains on the second region R2 after the execution of the step ST3. As a result of the execution of the step ST3, an upper opening UO is partially formed as can be seen from FIG. 6. The operation of each component of the plasma processing apparatus 10 can be controlled by the control unit Cnt.

In a step ST11 to be described later, the condition that realizes a deposition mode, i.e., a mode in which deposition of deposit containing fluorocarbon on the surface of the wafer W including the first region R1 dominates over the etching of the first region R1, is selected. On the other hand, in the step ST3, the condition that realizes an etching mode, i.e., a mode in which the etching of the first region R1 dominates over the deposition of deposit, is selected. Therefore, in this example, the fluorocarbon gas used in the step ST3 may contain at least either one of C₄F₈ gas and CF₄ gas. A ratio of fluorine atoms to carbon atoms (i.e., the number of fluorine atoms/the number of carbon atoms) in the fluorocarbon gas used in the step ST3 is higher than that in the fluorocarbon gas used in the step ST11. In this example, in order to increase a degree of dissociation of the fluorocarbon gas, the high frequency power for plasma generation used in the step ST3 may be set to be greater than that used in the step ST11. Accordingly, the etching mode can be realized. In this example, the high frequency bias power used in the step ST3 may be set to be greater than the high frequency bias power used in the step ST11. Therefore, the energy of ions attracted to the wafer W can be increased. As a result, the first region R1 can be etched at a high speed.

The step ST3 is executed under the following condition.

-   -   Pressure in processing chamber: 10 mTorr (1.33 Pa) to 50 mTorr         (6.65 Pa)     -   Processing gas         -   C₄F₈ gas: 10 sccm to 30 sccm         -   CF₄ gas: 50 sccm to 150 sccm         -   Ar gas: 500 sccm to 1000 sccm         -   O₂ gas: 10 sccm to 30 sccm     -   High frequency power for plasma generation: 500 W to 2000 W     -   High frequency bias power: 500 W to 2000 W

Next, a step ST4 is executed. In the step ST4, a plasma of a processing gas containing an oxygen-containing gas is generated in the processing chamber 12. To do so, in the step ST4, the processing gas is supplied into the processing chamber 12 from a gas source selected among the gas sources of the gas source group 40. In this example, the processing gas used in the step ST4 may contain oxygen gas as an oxygen-containing gas. Moreover, the processing gas may further include a rare gas (e.g., Ar gas) or an inert gas such as nitrogen gas. In the step ST4, a pressure in the processing chamber 12 is set to a predetermined level by the operation of the gas exhaust unit 50. In the step ST4, the high frequency power from the first high frequency power supply 62 is supplied to the upper electrode 30. In the step ST4, the high frequency bias power from the second high frequency power supply 64 may not be supplied to the lower electrode LE.

In the step ST4, active species of oxygen are generated. A width of an upper end portion of the opening MO of the mask MK is increased by the active species of oxygen. Specifically, an upper shoulder portion of the mask MK which forms the upper end portion of the opening MO is etched in a tapered shape as can be seen from FIG. 7. Accordingly, even if a deposit generated in the following steps is adhered to a surface defining the opening MO of the mask MK, the decrease in the width of the opening MO can be reduced. The operation of each component of the plasma processing apparatus 10 in the step ST4 can be controlled by the control unit Cnt.

In a step ST12 to be described later, the step ST12 is carried out to decrease a very small amount of deposit generated in each sequence, and there is a need to suppress excessive decrease of the deposit. On the other hand, the step ST4 is executed to increase the width of the upper end portion of the opening MO of the mask MK and the processing time of the step ST4 needs to be short.

The step ST4 is executed under the following condition.

-   -   Pressure in processing chamber: 30 mTorr (3.99 Pa) to 200 mTorr         (26.6 Pa)     -   Processing gas         -   O₂ gas: 50 sccm to 500 sccm         -   Ar gas: 200 sccm to 1500 sccm     -   High frequency power for plasma generation: 100 W to 500 W     -   High frequency bias power: 0 W to 200 W

Next, in the method MT, a sequence SQ is repeatedly executed to etch the first region R1. The sequence SQ includes a step ST11, a step ST12, and a step ST13 that are executed in that order.

In the sequence SQ, the step ST11 is first executed. In the step ST11, a plasma of a processing gas is generated in the processing chamber 12 where the wafer W is accommodated. To do so, in the step ST11, the processing gas is supplied into the processing chamber 12 from a gas source selected among the gas sources of the gas source group 40. The processing gas used in the step ST11 contains a fluorocarbon gas. The processing gas may further contain a rare gas, e.g., Ar gas. In the step ST11, a pressure in the processing chamber 12 is set to a predetermined level by the operation of the gas exhaust unit 50. In the step ST11, the high frequency power from the first high frequency power supply 62 is supplied to the upper electrode 30. Accordingly, the plasma of the processing gas containing a fluorocarbon gas is generated and dissociated fluorocarbon is deposited on the surface of the wafer W. As a result, a deposit DP is formed as shown in FIG. 8. The operation of each component of the plasma processing apparatus 10 in the step ST11 can be controlled by the control unit Cnt.

As described above, in the step ST11, the condition that realizes the deposition mode is selected. Therefore, C₄F₆ gas is used as the fluorocarbon gas in this example.

The step ST11 is executed under the following condition.

-   -   Pressure in processing chamber: 10 mTorr (1.33 Pa) to 50 mTorr         (6.65 Pa)     -   Processing gas         -   C₄F₆ gas: 2 sccm to 10 sccm         -   Ar gas: 500 sccm to 1500 sccm     -   High frequency power for plasma generation: 100 W to 500 W     -   High frequency bias power: 0 W

Next, the step ST12 of the method MT is executed. In the step ST12, a plasma of a processing gas containing an oxygen-containing gas and an inert gas is generated in the processing chamber 12. To do so, in the step ST12, the processing gas is supplied into the processing chamber 12 from a gas source selected among the gas sources of the gas source group 40. In this example, the processing gas used in the step ST12 contains oxygen gas as the oxygen-containing gas. Further, in this example, the processing gas contains, as the inert gas, a rare gas such as Ar gas. The inert gas may be nitrogen gas. In the step ST12, a pressure in the processing chamber 12 is set to a predetermined level by the operation of the gas exhaust unit 50. In the step ST12, the high frequency power from the first high frequency power supply 62 is supplied to the upper electrode 30. In the step ST12, the high frequency bias power from the second high frequency power supply 64 may not be supplied to the lower electrode LE.

In the step ST12, active species of oxygen are generated and the amount of deposit DP on the wafer W is appropriately decreased by the active species of oxygen, as can be seen from FIG. 9. As a result, the opening MO and the upper opening UO are prevented from being blocked by an excessive amount of deposit DP. In the case of the processing gas used in the step ST12, the oxygen gas is diluted with the inert gas and, thus, excessive removal of the deposit DP can be suppressed. The operation of each component of the plasma processing apparatus 10 in the step ST12 can be controlled by the control unit Cnt.

The step ST12 is executed under the following condition.

-   -   Pressure in processing chamber: 10 mTorr (1.33 Pa) to 50 mTorr         (6.65 Pa)     -   Processing gas         -   O₂ gas: 2 sccm to 20 sccm         -   Ar gas: 500 sccm to 1500 sccm     -   High frequency power for plasma generation: 100 W to 500 W     -   High frequency bias power: 0 W

In the present embodiment, the step ST12 in each sequence, i.e., a single step ST12, is executed for two or more seconds. In the step ST12, the deposit DP can be etched at a rate of about 1 nm/sec or less. In the case of executing the sequence by using the plasma processing apparatus 10, it takeks time to switch gases when the steps ST11 to ST13 are shifted from one to another. Therefore, the step ST12 needs to be executed for two or more seconds in consideration of time required for stabilization of discharge. However, if the etching rate of the deposit DP in the step ST12 is too high, the deposit for protecting the second region R2 may be excessively removed. To that end, the deposit DP is etched at a rate of about 1 nm/sec or less in the step ST12. As a consequence, it is possible to appropriately control the amount of the deposit DP on the wafer W. The etching rate of the deposit DP which is about 1 nm/sec or less in the step ST12 can be realized by selecting a pressure in the processing chamber, a degree of dilution of oxygen in the processing gas with a rare gas, i.e., an oxygen concentration, and a high frequency power for plasma generation from the above-described condition.

Next, in the step ST13, the first region R1 is etched. To do so, in the step ST13, the processing gas is supplied into the processing chamber 12 from a gas source selected among the gas sources of the gas source group 40. The processing gas used in the step ST13 contains an inert gas. In this example, the inert gas may be a rare gas such as Ar gas. Or, the inert gas may be nitrogen gas. In the step ST13, a pressure in the processing chamber 12 is set to a predetermined level by the operation of the gas exhaust unit 50. In the step ST13, the high frequency power from the first high frequency power supply 62 is supplied to the upper electrode 30. In the step ST13, the high frequency bias power from the second high frequency power supply 64 is supplied to the lower electrode LE.

The step ST13 is executed under the following condition.

-   -   Pressure in processing chamber: 10 mTorr (1.33 Pa) to 50 mTorr         (6.65 Pa)     -   Processing gas         -   Ar gas: 500 sccm to 1500 sccm     -   High frequency power for plasma generation: 100 W to 500 W     -   High frequency bias power: 20 W to 300 W

In the step ST13, a plasma of an inert gas is generated and ions are attracted to the wafer W. The first region R1 is etched by radicals of fluorocarbon contained in the deposit DP. Accordingly, the first region R1 in the recess formed by the second region R2 is etched and a lower opening LO is gradually formed, as can be seen from FIG. 10. The operation of each component of the plasma processing apparatus 10 in the step ST13 can be controlled by the control unit Cnt.

In the method MT, the sequence SQ including the above-described steps ST11 to ST13 is repeated. As the sequence SQ is repeated, the deposit DP is formed on the wafer W by the execution of the step ST11 as can be seen from FIG. 11. Then, the amount of the deposit DP is decreased by the execution of the step ST12 as can be seen from FIG. 12. Next, the first region R1 is further etched by the execution of the step ST13 and, thus, the depth of the lower opening LO is increased, as can be seen from FIG. 13. As the sequence SQ is further repeated, the deposit DP is formed on the wafer W by the execution of the step ST11, as can be seen from FIG. 14. Thereafter, the amount of the deposit DP is decreased by the execution of the step ST12 as can be seen from FIG. 15. Next, the first region R1 is further etched by the execution of the step ST13 and, thus, the depth of the lower opening LO is further increased, as can be seen from FIG. 16. Ultimately, the first region R1 is etched until the second region R2 is exposed at the bottom of the recess.

Referring back to FIG. 1, it is determined in the step STa of the method MT whether or not a stop condition is satisfied. When the sequence SQ has been performed a predetermined number of times, it is determined that the stop condition is satisfied. When it is determined in the step STa that the stop condition is not satisfied, _(t)he sequence SQ is performed again from the step ST11. On the other hand, when it is determined in the step STa that the stop condition is satisfied, the execution of the method MT is terminated.

In the present embodiment, the conditions of the repeated sequences SQ may be set such that an etching amount of the first region R1 in the sequence SQ performed during a period including a time when the second region R2 is exposed (hereinafter, referred to as “first sequence”) becomes smaller than that of the first region R1 in a next sequence SQ (hereinafter, referred to as “second sequence”). In this example, the execution time of the first sequence is set to be shorter than that of the second sequence. In this example, a ratio of the execution time of the steps ST11, ST12 and ST13 in the first sequence may be set to be equal to that of the steps ST11, ST12 and ST13 in the second sequence. For example, in the first sequence, the execution time of the step ST11 is selected from a range of 2 sec to 5 sec; the execution time of the step ST12 is selected from a range of 2 sec to 5 sec; and the execution time of the step ST13 is selected from a range of 5 sec to 10 sec. In the second sequence, the execution time of the step ST11 is selected from a range of 2 sec to 10 sec; the execution time of the step ST12 is selected from a range of 2 sec to 10 sec; and the execution time of the step ST13 is selected from a range of 5 sec to 20.

Although the active species of fluorocarbon generated in the step ST11 are deposited on the second region R2 and protect the second region R2, the second region R2 may be etched when the second region R2 is exposed by the etching process of the first region R1. Therefore, in the present embodiment, the first sequence is performed during a period in which the second region R2 is exposed. Accordingly, the deposit DP is formed on the wafer W while decreasing the etching amount and the second region R2 is protected by the deposit DP. Then, the second sequence in which the etching amount is large is performed. As a result, in the present embodiment the first region R1 can be etched while suppressing the etching of the second region R2.

In the step ST13 of the sequence SQ (hereinafter, referred to as “third sequence”) executed after the second sequence, the high frequency bias power may be set to be greater than the high frequency power used in the step ST13 of the first sequence and the second sequence. For example, in the step ST13 of the first sequence and the second sequence, the high frequency bias power is set within a range from 20 W to 100 W. In the step ST13 of the third sequence, the high frequency bias power is set within a range from 100 W to 300 W. In the third sequence of this example, the execution time of the step ST11 is selected from a range of 2 sec to 10 sec; the execution time of the step ST12 is selected from a range of 2 sec to 10 sec; and the execution time of the step ST13 is selected from a range of 5 sec to 15 sec.

As can be seen from FIG. 14, the amount of the deposit DP on the wafer W is considerably increased after the execution of the first sequence and the second sequence. When the amount of the deposit DP is increased, the width of the opening MO, the width of the upper opening UO, and the width of the lower opening LO are decreased by the deposit DP. Accordingly, the flow velocity of ions may not be enough to reach the deep portion of the lower opening LO. Since, however, a relatively large high frequency bias power is used in the step ST13 of the third sequence, the energy of ions attracted to the wafer W can be increased. As a result, even if the lower opening LO has a large depth, ions can reach the deep portion of the lower opening LO.

Hereinafter, an etching method according to another embodiment will be described. FIG. 17 is a flowchart of the etching method according to another embodiment. FIGS. 18 and 19 are cross sectional views of the target object after the execution of a step ST14 of the method shown in FIG. 17. FIG. 18 shows the state of the cross section of the wafer W after the wafer W shown in FIG. 10 is subjected to the step ST14. FIG. 19 shows the state of the cross section of the wafer W after the wafer W shown in FIG. 13 is subjected to the step ST14. The method MT2 shown in FIG. 17 is different from the method MT in that the sequence SQ further includes the step ST14 executed after the step ST13. The step ST14 is equal to the step ST12. As for the processing condition of the step ST14, the processing condition of the step ST12 may be employed.

In the step ST13, ions are attracted to the wafer W as described above. Therefore, a material forming the deposit DP is emitted from the wafer W and adhered to the wafer W again. Accordingly, the deposit DP is formed so that the widths of the opening MO and the lower opening LO are decreased, as can be seen from FIGS. 10 and 13. The deposit DP may block the opening MO and the lower opening LO in some cases. In the method MT2, the wafer W shown in FIGS. 10 and 13 is exposed to active species of oxygen by the execution of the step ST14 as in the case of executing step ST12. As a consequence, the deposit DP that narrows the widths of the opening MO and the lower opening LO (see FIGS. 10 and 13) can be decreased as can be seen from FIGS. 18 and 19. As a result, it is possible to reliably prevent the blockage of the opening MO and the lower opening LO.

While various embodiments have been described, the disclosure may be variously modified without being limited to the above-described embodiments. For example, in _(t)he method MT, the high frequency power for plasma generation is supplied to the upper electrode 30. However, the high frequency power may be supplied to the lower electrode LE. Further, in the method MT, a plasma processing apparatus other than the plasma processing apparatus 10 may be employed. Specifically, the method MT can be performed by any plasma processing apparatus such as an inductively coupled plasma processing apparatus or a plasma processing apparatus in which a plasma is generated by using a surface wave such as a microwave.

The execution order of the steps ST11 to ST13 in the sequence SQ of the method MT may be changed. For example, in the sequence SQ of the method MT, the step ST12 may be executed after the step ST13.

While the disclosure has been shown and described with respect to the embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the disclosure as defined in the following claims. 

What is claimed is:
 1. A method for selectively etching a first region of silicon oxide with respect to a second region of silicon nitride by performing plasma processing on a target object including the second region formed to have a recess, the first region provided to fill the recess and to cover the second region, and a mask provided on the first region, the method comprising the steps of: (a) generating a plasma of a processing gas containing a fluorocarbon gas in a processing chamber where the target object is accommodated and forming a deposit containing fluorocarbon on the target object; (b) generating a plasma of a processing gas containing an oxygen-containing gas and an inert gas in the processing chamber; and (c) etching the first region by radicals of fluorocarbon contained in the deposit, wherein a sequence including the step (a), the step (b) and the step (c) is repeatedly performed.
 2. The method of claim 1, wherein the mask is made of an organic material, wherein an anti-reflection film containing silicon is provided on the mask, wherein the method further comprises the steps of: (d) generating a plasma of a processing gas containing a fluorocarbon gas in the processing chamber and etching the first region until immediately before the second region is exposed; and (e) generating a plasma of a processing gas containing an oxygen-containing gas in the processing chamber, and wherein the sequence is performed after executing the step (d) and the step (e).
 3. The method of claim 1, wherein the step (b) is executed for two or more seconds and the deposit is etched at a rate of 1 nm/sec or less in the step (b).
 4. The method of claim 1, wherein in the sequence, the step (b) is performed between the step (a) and the step (c), and wherein the sequence further includes another step of generating a plasma of a processing gas containing an oxygen-containing gas and an inert gas in the processing chamber where the target object is accommodated.
 5. The method of claim 2, wherein in the sequence, the step (b) is performed between the step (a) and the step (c), and wherein the sequence further includes another step of generating a plasma of a processing gas containing an oxygen-containing gas and an inert gas in the processing chamber where the target object is accommodated. 